Work Done & Energy Transfer (AQA GCSE Physics): Revision Notes
Work and energy
What is work done?
Work done happens when a force moves an object through a distance. Think of it as the energy you use up when you push, pull, or lift something.
Key fact: Work done by a force equals the energy transferred.
When you do work on an object, you transfer energy to it. This is a fundamental principle in physics that helps us understand how energy moves from one place to another. For example:
- Pushing a box across the floor transfers energy to the box
- Lifting a book transfers energy to increase its height
The energy you transfer when doing work doesn't disappear - it gets converted into other forms like kinetic energy (movement) or potential energy (stored energy due to position).
The work done formula
You can calculate work done using this simple formula:
Work done = Force × Distance moved in the direction of the force
Where:
- W = work done (measured in joules, J)
- F = force (measured in newtons, N)
- s = distance moved (measured in metres, m)
Important unit fact: 1 joule = 1 newton-metre
This means if a 1 N force moves an object 1 m, then 1 J of work is done.
Work done when moving horizontally
When you push something along the ground, you're doing work against friction. The amount of work depends on two key factors:
- The harder you push (more force), the more work you do
- The further you push it (more distance), the more work you do
Work done against friction makes objects heat up because the energy gets converted into thermal energy. This is why your hands get warm when you rub them together quickly!
Example: Pushing a car along a road - your muscles do work against the friction between the tyres and road.
Work done when lifting objects
When you lift something up, you do work against gravity. This increases the object's gravitational potential energy.
The work done when lifting can be calculated using either formula:
Where:
- m = mass of object (kg)
- g = gravitational field strength (10 N/kg on Earth)
- h = height lifted (m)
Worked Example: Lifting a Box
Problem: Calculate the work done when lifting a 20 kg box up 1.5 m.
Step 1: Identify the known values
- Mass (m) = 20 kg
- Height (h) = 1.5 m
- Gravitational field strength (g) = 10 N/kg
Step 2: Apply the formula Work done = m × g × h
Step 3: Substitute and calculate Work done = 20 kg × 10 N/kg × 1.5 m = 300 J
Simple examples
Here are two straightforward examples showing different types of work:
Worked Example: Pushing Horizontally
A force of 100 N pushes an object 3 m horizontally.
Step 1: Apply the formula W = F × s Step 2: Substitute values Work done = 100 N × 3 m = 300 J
Worked Example: Lifting Vertically
A 20 kg object is lifted 1.5 m vertically.
Step 1: Apply the formula W = m × g × h
Step 2: Substitute values
Work done = 20 kg × 10 N/kg × 1.5 m = 300 J
Key Points to Remember:
- Work done = Force × Distance ()
- Work done equals energy transferred
- Units: joules (J) = newton-metres (N⋅m)
- Pushing against friction heats things up
- Lifting against gravity increases potential energy